1. Field of the Invention
The present invention relates to hydrogenated copolymers of butadiene and at least one other diene, preferably isoprene. More particularly, it relates to block or segmented hydrogenated copolymer of butadiene and at least one other diene, preferably isoprene, containing at least one crystallizable segment or block comprising an average of at least about 10 weight percent of the total hydrogenated copolymer chain, and at least one low crystallinity segment or block. The instant invention also relates to oleaginous compositions containing said copolymers as viscosity index improver additives, a process for making these copolymers and a method to control the viscosity of oleaginous compositions.
2. Description of Related Art
Various copolymers of butadiene with other olefins are known to be used as oil additives. These include hydrogenated copolymers of butadiene with another conjugated diene such as isoprene. The copolymers are disclosed to be random or block copolymers. The following review of the related art illustrates attempts which have been made to develop hydrogenated butadiene base copolymers for use as an oil additive including a viscosity modifier.
U.S. Pat. No. 4,804,794 discloses segmented copolymers of ethylene and at least one other .alpha.-olefin monomer. Each copolymer is intramolecularly heterogeneous and intramolecularly homogeneous. At least one segment of the copolymer constituting at least 10% of the copolymer chain, is a crystallizable segment. The remaining segments of the copolymer chain are termed low crystallinity segments, and are characterized by an ethylene content of not greater than about 53 weight percent.
The .alpha.-olefin can include those containing 3 to 18 carbon atoms. .alpha.-olefins having 3 to 6 carbon atoms are indicated to be preferred. With the most preferred copolymers being copolymers of ethylene with propylene or ethylene with propylene and diene. The copolymers are disclosed to improve the properties in oleaginous fluids, in particular lubricating oil.
U.S. Pat. No. 3,419,365 discloses hydrogenated copolymers of butadiene and styrene as pour point depressants for distillate fuel oil; U.S. Pat. No. 3,393,057 discloses polymers of butadiene C10 to C24 normal alpha-monoolefins and styrene or indene as pour point depressants for fuel and lubricating oils; and U.S. Pat. No. 3,635,685 discloses pour point depressants comprising hydrogenated butadiene-styrene copolymers which contain a hydroxy, carboxy, or pyridyl terminal group.
U.S. Pat. No. 3,312,621 discloses polymers of conjugated diolefins which are predominantly in the 1,4-addition configuration, as viscosity index (V.I ) improvers. Butadiene, isoprene, 1,3-pentadiene, and copolymers of such diolefins are specifically disclosed as suitable.
U.S. Pat. No. 3,600,311 discloses viscosity index improvers of hydrogenated homopolymers of butadiene in which about 45 to 95 percent of the butadiene monomers are in the 1,4-configuration.
U.S. Pat. No. 3,795,615 discloses viscosity index improvers of hydrogenated copolymers of butadiene with a different conjugated diene, e.g., isoprene in which the monomer units in the polymer are predominantly in the 1,4-configuration.
U.S. Pat. No. 3,965,019 discloses hydrogenated random, tapered or block copolymers of butadiene and isoprene to be useful as viscosity modifiers.
U.S. Pat. No. 4,032,459 discloses a viscosity index improver comprising a copolymer of butadiene and isoprene having between 20-55% 1,4-configuration, this polymer then having been hydrogenated to remove substantially all of the olefinic unsaturation.
U.S. Pat. No. 4,073,737 discloses viscosity index improver comprised of a hydrogenated copolymer produced by copolymerization of from about 1 to about 10 mole percent butadiene, at least one other C5 to C12 conjugated diene, and up to 45 mole percent of a vinyl aromatic monomer.
A useful class of viscosity index improvers for lube oil compositions are star-shaped polymers comprising a nucleus such as divinylbenzene with polymeric arms linked to it. Such polymers are disclosed in patents, such as U.S. Pat. No. 4,358,565 and 4,620,048. Generally star-shaped polymers are disclosed to be formed by polymerizing one or more conjugated dienes and optionally, one or more monoalkenylarene compound in solution in the presence of an ionic initiator to form a living polymer. For the purpose of the present invention the term "living polymer" is used consistent with Billmeyer, Textbook of Polymer Science, 2d Ed., Wiley-Interscience, John Wiley and Sons, page 318, (1971). Specific conjugated dienes include conjugated dienes having 4 to 12 carbon atoms and optionally, one or more monoalkenylarene compounds. Typical and useful conjugated dienes include butadiene (1,3-butadiene) and isoprene. The living polymers thereby produced are then reacted with a polyalkenyl coupling agent to form star-shaped polymers. The coupling agents have at least two non-conjugated alkenyl groups. The groups are usually attached to the same or different electron drawing groups, e.g., an aromatic nucleus. Such compounds have the property that at least two of the alkenyl groups are capable of independent reaction with different living polymers, and in this respect, are different from conventional dienes polymerizable monomers, such as butadiene and isoprene. The coupling agents may be aliphatic, aromatic or heterocyclic. Examples of aliphatic compounds include polyvinyl and polyallyl acetylenes, diacetelenes, phosphates, and phosphites, as well as the dimethacrylates, e.g., ethyl dimethacrylate. Examples of suitable heterocyclic compounds include divinyl pyridine and divinyl thiophene. Coupling agents disclosed to be preferred in the U.S. Pat. No. 4,358,565 patent are polyalkenyl aromatic compounds with the most preferred being indicated to be polyvinyl aromatic compounds. Examples of such compounds include those aromatic compounds, e.g., benzene, toluene, xylene, anthracene, naphthalene, and durene which are substituted by at least two alkenyl groups, preferably directly attached thereto. Specific examples include polyvinyl benzenes, e.g., divinyl, trivinyl, and tetravinyl benzenes; divinyl, trivinyl and tetravinyl, autho, meta and paraxylenes, divinyl naphthalene, divinyl ethyl benzene, divinyl biphenyl, diisobutenyl benzene, diisopropanol benzene, and diisopropanol biphenyl. It is disclosed that the polyalkenyl coupling agent should be added to the living polymer after the polymerization of the monomers is substantially complete, i.e., the agent should only be added after substantially all of the monomer has been converted to living polymer.
The amount of polyalkenyl coupling agent added may vary, but preferably, at least 0.5 moles is used per mole of unsaturated living polymer. Amounts from 1 to 15 moles, and preferably 1.5 to 5 moles are preferred.
There exists a need for viscosity index improvers which, when added to oleaginous compositions, such as lube oil compositions, yield compositions exhibiting better or improved low temperature viscometric characteristics than are obtainable by the use of conventional viscosity index improver additives. The copolymers of the instant invention provide oleaginous compositions exhibiting such improved low temperature viscometric characteristics, and additionally, improved shear stability.
For convenience, certain terms that are repeated throughout the present specification and claims are defined below.
a. Viscosity Index (V.I. is the ability of a lubricating oil to accommodate increase in temperature with a minimum decrease in viscosity. The greater this ability, the higher the V.I.
b. A block copolymer is a copolymer having at least one sequence (also referred to as block or segment) of the same monomer units. Each sequence has at least two monomer units. Block copolymers typically have a plurality of each type of monomer making up the copolymer. The terms relating to block copolymer are consistent with those given in Billmeyer, Jr., Textbook Polymer Science, Second Edition, Wiley-Interscience (1971).
c. Average methylene content of a segment of the hydrogenated copolymer is the average number of methylene moieties or units present in the segment. The methylene units are those present in the particular segment as a result of the polymerization and hydrogenation of butadiene and at least one other conjugated diene, such as isoprene. Thus, for example, the hydrogenation product of the 1,4-addition product of two butadiene molecules contains one methylene segment comprised of eight methylene units and has a methylene content of 100%, i.e., --CH.sub.2 --CH.sub.2 --CH.sub.2 --CH.sub.2 --CH.sub.2 --CH.sub.2 --CH.sub.2 --CH.sub.2 --. The hydrogenation product of the 1,4-addition product of three isoprene molecules contains 9 methylene units, 3 substituted methylene units, includes two methylene sequences containing three methylene units each and one methylene sequence containing two methylene units, and has a methylene content of 75 mole percent, i.e., ##STR1##
The hydrogenation product of the 1,4-addition product of one molecule of butadiene and one molecule of isoprene includes one methylene sequence comprised of 6 methylene units, contains 7 methylene units and one substituted methylene unit, and contains 87.5 mole percent methylene units, i.e., ##STR2##
The following defines how percent methylene units are calculated from the percentages of hydrogenated 1,2 and 1,4 polybutadiene in the polymer. Each enchained 1,4-butadiene (B.sub.1,4) unit contributes four methylenes. Each 1,2-butadiene contributes one methylene and one substituted methylene. Where B represents the mole fraction 1,4-butadiene then (1-B) is the fraction 1,2-butadiene. The mole fraction methylenes present, X.sub.CH.sbsb.2 is then ##EQU1##
For the homopolymer polybutadiene, B=1 and the fraction X.sub.CH.sbsb.1 =1. For a homopolymer of isoprene, B=O, and X.sub.CH.sbsb.2 =0.75. If it is desired to interpret this composition in terms of ethene units X.sub.CH.sbsb.2 CH.sub.2 the result is ##EQU2##
For B=1, X.sub.CH.sbsb.2 CH.sub.2 =1 whereas for B=O, X.sub.CH.sbsb.2 CH.sub.2 =1/2. A hydrogenated isoprene polymer is in effect an alternating ethene and propene copolymer. By definition hydrogenated isoprene methylenes are not considered to be 3/2 ethylene but only as supplying a single ethylene unit.
Depending on whether the butadiene is incorporated as blocks or as a statistical copolymer with the isoprene, different percentages of methylene units result in crystallinity.
In all cases where sequences of methylenes are long enough to crystallize at least two adjacent hydrogenated 1,4-butadiene units will be present, and generally 3 or more will be present. In such cases whether methylenes from isoprene are counted or not becomes less important, since isoprene adds only one or two units to an 8 to 12 unit sequence. For the purposes of the present invention the composition of crystallizable segments are considered to be in terms of butadiene content, with only those methylene segments contributed by butadiene units counted. Thus, EQU X.sub.CH.sbsb. =B (3)
If there is no butadiene present there are no crystallizable methylenes. For B=1 all methylenes are crystallizable. For B=0.5 half the methylenes are crystallizable. As noted below, not all methylenes that are crystallizable will crystallize.
For a random copolymer of butadiene and isoprene about 3% of the methylenes present (from butadiene) will be in sequences of 20 or more at B0.3; at B=0.4 about 9%; and at B=0.5 the value is about 19%. (C. Tosi, Advances in Polymers Science, 5, 451 (1968)).
For B0.3, 0.4, 0.5, X.sub.CH.sbsb.2 =0.82, 0.85, 0.875 respectively (Formula 1). It is through this region of butadiene composition, 30, 40 50%, that semicrystalline polymers are formed in statistical copolymers.
For block as opposed to random copolymers lower amounts of butadiene needed for crystallizable segments, and in a butadiene rich block the butadiene content can equal 100%. If the butadiene is mixed with isoprene in the crystallizable segment but is "blocked", then crystallinity could appear only marginally above the X.sub.CH.sbsb. =0.75 value for hydrogenated polyisoprene given by formula (1).
The definitions above pertain to exclusive 1,4-butadiene monomer addition. In the case of 1,2- and 1,4-butadiene addition similar definitions can be made. Enchained 1,2-butadiene produces one methylene group and one substituted methylene. The overall mole fraction of methylene groups is ##EQU3##
B.sub.1,4 is the fraction of 1,4-addition of butadiene. For B.sub.1,4 =1, XCH=1. For all 1,2-butadiene addition X.sub.CH.sbsb.2 =0.5, there can be no long methylene sequences.
To obtain crystallizable sequences there must be sequences of 1,4-butadiene enchained units. For random addition of 1,2-butadiene and 1,4-butadiene units, B.sub.1,4 is approximately 0.3 (i.e., 30 percent) to obtain crystallinity. From formula (4) XCH =0.73 at B.sub.1,4 =0.3. If the statistical distribution of 1,2-butadiene and 1,4-butadiene additions is of a block type, than random crystallinity could occur at B.sub.1,4 below 0.3.
For polymers containing styrene or another monomer which introduce only two carbons to the polymer backbone, the methylene content can be reduced to only 0.5 mole fraction. Thus, for polymers containing such monomers or significant 1,2-butadiene addition or 3,4-isoprene addition, the percentage of methylene units present can be lower than 0.75. The precise sequence distribution determines the lower amount of methylene content that can be crystallizable. The fraction of methylenes present in the case of polymers containing 1,4- and 1,2-butadiene and 1,4- and 3,4-isoprene is ##EQU4##
B.sub.1,2 is the fraction of the total polymer formed from 1,2-addition of butadiene, I.sub.1,4 the fraction of 1,4-addition isoprene; and I.sub.3,4 the fraction of 3,4 addition of isoprene. B.sub.1,4 is the most important variable for crystallizable methylenes.
The substituted methylene content is calculated as ##EQU5## Where there is no B.sub.1,4 or I.sub.1,4 present this value can have a maximum of 0.5. When B.sub.1,4 is less than 0.2 the substituted fraction is large.
d. By "crystallize" it is meant that the methylene sequences in the polymer associate into ordered state consistent with the classical definition of polymer crystallinity as set forth, for example, by Flory, Principles of Polymer Chemistry, Cornell University Press (1953).
e. Crystallizable units are defined as methylene groups in sequences which exhibit a heat of fusion when measured by differential scanning calorimetry (DSC) upon cooling from the melt. In a useful procedure for the present invention a sample is formed into an approximately 0.030 inch thick sheet for 30 minutes at 150.degree. C. and is then annealed at 20.degree. C. for 48 hours prior to measurement, loaded into the calorimeter at that temperature, rapidly cooled to -100.degree. C. and scanned to 150.degree. C. at 20.degree. C./minute. Only sequences melting between 20.degree. C. and 140.degree. C. are included.
f. The weight percent crystallizable units is: ##EQU6##
On this basis pure melt crystallized polymethylene of high MW (i.e., where the end group effects are not significant; typically greater or equal to about 20,000) has about 60% crystallizable units. Kinetic restrictions prevent them from all crystallizing. Percent crystallinity can be measured by techniques, as defined in G. Ver Strate, Z. W. Wilchinsky, J. Pol. Sci Physics, A2, 9, 127 (1971), which is incorporated herein by reference. The degree of crystallinity measured is a function of the sample's annealing history. Some low amount is desirable in this product when the sample is annealed at 20.degree. C. for more than 48 hours after preparation of a void-free, strain-free specimen by heating to 150.degree. C. for 30 minutes in a suitable mold. Crystallizability also depends on other factors: temperature, diluent, and the composition of the copolymer.
g. The association temperature (T.sub.a), is the temperature at which crystallization of the copolymer of the present invention is determined by studying the temperature dependance of the relative viscosity (.eta.rel). Deviation from an established d.eta.rel/dT) trend occurs when significant association at polymer segments due to crystallinity starts. (ASTM method D-445 for kinematic viscosity can be run at a series of temperatures. The polymer concentration in these measurements should be the same as that in the formulated oil, for example 1%).
h. The cloud point temperature is the temperature (T.sub.c) at which crystalline clouds (turbidity) are first observable upon cooling of oil when tested according to ASTM D-2500. The cloud point temperature can be correlated with the association of temperature.
i. A crystallizable segment of the hydrogenated copolymer chain is rich in methylene units, with an average methylene content of at least about 75 mole percent. The methylene content depends on the comonomer used to prepare the polymer and the nature of their inclusion in the polymer. The methylene units will crystallize at a given temperature and concentration in solution only if they are in long enough sequences, with only a limited number of interruptions due to substituted methylene units. More interruptions are acceptable with methyl substitutions than with larger groups since methyl groups can enter the polymethylene crystal lattice. Methylene units can be identified by C.sub.13 NMR, T. Hayashi, Y. Iroue, R. Chujo, Macromolecules, 21, 3139, 1988, and references therein. Sequences of 5 or more cannot be distinguished. However units are actually crystallizable only if they are present in sequences of about 11 methylenes or longer.
j. A low crystallinity segment has an average methylene content less than about 75 mole percent, and is characterized in the unoriented bulk state after at least 24 hours annealing by a degree of crystallinity of less than about 0.2% at 23.degree. C.
k. Molecular weights of the hydrogenated copolymer were measured by a combination of gel permeation chromatography and on line laser light scattering as described in G. Ver Strate C. Cozewith, S. Ju, Macromolecules, 21, 3360, 1988. The specific refractive index increment in trichlorobenzene at 135.degree. C. was assumed to be -0.104 for all hydrocarbon structures.